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Fish reared on land replay the transition to four legs

By Andy Coghlan

Bichir fish raised on land became better walkers

(Image: Antoine Morin)

It was one of the key events in the evolution of animals – and now it has been replayed in the lab. Evolutionary biologists reared air-breathing fish on land for eight months and found that the experience encouraged the fish to develop skeletons better adapted for walking.

Fish first adapted to life on land about 350 to 400 million years ago, when they evolved four legs to form the tetrapods, a group that includes amphibians, reptiles, birds and mammals. This crucial evolutionary transition is preserved in the fossilised remains of animals called stem tetrapods, which have some features of fish and some of four-legged animals.

Bichir fish (Polypterus), which are native to Africa’s Nile basin, are the closest living counterparts of those ancient stem tetrapods. They have true lungs as well as gills, so they can survive on land as well as in their preferred underwater habitat. For these reasons, Emily Standen at the University of Ottowa in Canada and her colleagues decided to try rearing them on land to see whether this made them more adept at walking than bichir fish that were raised exclusively in water.

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The short answer is that this is exactly what happened. “The anatomy of the fish raised on land changed, and those changes reflected what we see in the fossil record in the transition from fish to four-legged terrestrial vertebrates,” says Standen.

Video: Walking fish replays evolution from land to sea

Fin by fin

Bichir fish, which resemble eels, normally “walk” by alternately planting each of their two front fins firmly on the sand. In each “step”, they use one fin as an anchor for wiggling the rear parts of their body forward, ready for the next “step” with the other fin (see video).

This walking technique improved in 100 bichir fishes that Standen and her colleagues reared in a tank filled with just a couple of millimetres of water. They doused the fish constantly with mist to stop them drying out and dying.

These fish – and another 50 in a standard water-filled aquarium – were reared for eight months, after which Standen’s team tested the walking capabilities of both groups. The researchers filmed the fish for finer analysis of their walking, and examined bones and muscles to see whether there were anatomical differences between the two groups.

“The main outcome was that if Polypterus were raised on land, behaviourally they walked more effectively,” says Standen.

Quick evolution

The land-raised fish raised their heads 1.5 times higher on average during walking to reinforce forward momentum, reduced the slippage of their “anchor” fins by a third and planted their fins 3.5 times closer to the midlines of their bodies, providing superior leverage for the rear part of the body.

Their bodies also changed in a way that made them more like stem tetrapods. Their shoulder blades grew slightly longer than normal and were in better contact with a neighbouring bone called the cleithrum. Both of these changes were seen in stem tetrapods as they moved onto land.

There were also changes in bone structures that gave the land-living bichir something resembling the beginnings of a neck. In stem tetrapods, the neck ultimately separated the head from the body and is seen in today’s terrestrial animals.

“This data is the first to present evidence that environmentally induced developmental plasticity may have been present in the stem tetrapods, helping facilitate their successful transition onto land,” says Standen.

“This research may show us one of the influences acting during the crucial water-to-land transition in tetrapods,” says Jenny Clack of the University of Cambridge. “But I’m not sure how it could be tested further or refuted,” she adds.

Standen says that ideally, she’d like to study multiple generations of Polypterus to see whether the traits are reinforced over time, but she notes that these fish are notoriously difficult to breed. “We always have breeding as a goal on the horizon, and are continuing to raise fish on land to examine other potential changes that a new environment might induce.”